This invention relates to a microwave generator called a turbutron and more particularly to an apparatus and method for producing a high power gigawatt level source of millimeter-wave radiation.
There are available in the prior art other types of high power microwave sources that utilize an electron beam as a source of free energy with a resonant structure to convert the electron beam free energy to oscillatory electromagnetic energy. There are also devices including reflex triodes, reflex diodes, and vircators which utilize the concept of a virtual cathode to generate microwaves. Each of these devices has a boundary or virtual cathode beyond the primary anode plane through which relatively few electrons penetrate and from which most electrons are reflected.
The reflex triode has been demonstrated to be an impressive high power source of microwave radiation. In particular ten nanosecond bursts of gigawatt level microwave radiation were achieved in X-band at 10 GHz. The work is documented in the Harry Diamond Laboratories technical report HDL-TR-1917, dated Aug., 1980 and entitled, Gigawatt Microwave Emission from a Relativistic Reflex Triode, by H. E. Brandt, A. Bromborsky, H. B. Bruns, R. A. Kehs, and G. P. Lasche. In the reflex triode the cathode and plate are at ground potential and the anodal grid is at a high megavolt level voltage. An intensely oscillating relativistic electron plasma is created in and beyond the accelerating gap and is the source of microwaves. Most of the electrons are reflected at the virtual cathode. Diode and triode configurations do not differ significantly in their dynamical and radiation characteristics. In particular, a diode configuration may be modeled analogously to the triode, but with the plate and grid at equal positive megavolt potential with respect to the cathode. In the diode a virtual cathode forms at nearly the same position as for the triode. In both devices the electrons are concentrated within approximately 1 cm of both sides of the grid for a 1 cm gap, and the dominant spectral characteristics are comparable. Both the level and the frequency content of the power spectral densities are very similar. In the preferred embodiment the turbutron has such a diode configuration but with a 3 mm gap instead of a 1 cm gap in order to increase the dominant frequency from 10 GHz to the atmospheric window at 35 GHz. The earlier success with the reflex triode in both predicting and obtaining gigawatt power levels at 10 GHz, together with the fact that diode and triode configurations have comparable microwave characteristics, were the bases for the turbutron concept.
The vircator is essentially a reflex diode in which the reflexing electrons are removed in some way, for example, by shaping the confining magnetic field or employing a slotted anode. The remaining electrons are concentrated in a potential well localized at the virtual cathode and oscillating in a single mode. The operation of the vircator requires the frequency of the oscillating virtual cathode to be the same as that of a mode of an axial waveguide/drift tube so as to be at resonance. The turbutron, however, produces gigawatt radiation without the requirements of a tuned axial drift tube, or magnetic focusing of the beam, and without wasting a significant fraction of the electron beam. The turbutron is not as complicated as the vircator nor does it require the precision tuning of the vircator. In addition the turbutron diode configuration has allowed for the increase of the dominant frequency to the atmospheric window at 35 GHz which is a significant improvement over high power reflex triodes and diodes which operate in the 10 GHz range. These features can be particularly important in military applications.